Argentine leader's power at stake in Congress vote

People line up to vote at a polling station during mid term legislative elections in Buenos Aires' Tigre district , Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Natacha Pisarenko)
— AP

People line up to vote at a polling station during mid term legislative elections in Buenos Aires' Tigre district , Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Natacha Pisarenko)
/ AP

A police officer helps a man to look for his name in electoral rolls at a polling station, during mid term legislative elections in Buenos Aires, Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Eduardo Di Baia)— AP

A police officer helps a man to look for his name in electoral rolls at a polling station, during mid term legislative elections in Buenos Aires, Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Eduardo Di Baia)
/ AP

Women look for their names in electoral rolls at a polling station during mid term legislative elections in Buenos Aires' Tigre district , Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Natacha Pisarenko)— AP

Women look for their names in electoral rolls at a polling station during mid term legislative elections in Buenos Aires' Tigre district , Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Natacha Pisarenko)
/ AP

A man enters to vote at a polling station during mid term legislative elections in Buenos Aires' Tigre district, Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Natacha Pisarenko)— AP

A man enters to vote at a polling station during mid term legislative elections in Buenos Aires' Tigre district, Argentina, Sunday, Oct. 27, 2013. Sunday's run-up to congressional elections will decide how much control President Cristina Fernandez will have over Argentine politics during the final two years of her presidency. (AP Photo/Natacha Pisarenko)
/ AP

BUENOS AIRES, Argentina 
President Cristina Fernandez's ruling Front for Victory lost ground in Argentina's congressional elections Sunday, giving up seats in the four largest districts.

Her former Cabinet chief and now political rival, Sergio Massa, won the most votes of any politician in the country, one of his allies predicted.

It was too early to tell based on exit polling alone whether the government and its allies lost the thin majorities they've used in the lower house and Senate to dominate the country's political agenda. The first official results were expected sometime Sunday night.

Juliana di Tullio, a ruling bloc candidate for the lower house, said the government believed it wouldn't lose its majorities in either chamber.

However, Massa's arrival in Congress, as a popular rival with a slate of followers claiming to represent the center in politically polarized Argentina, represents a new threat to Fernandez's all-or-nothing style of governing.

And because Fernandez clearly failed to gain the two-thirds majorities in both houses needed to end constitutional term limits, the vote also ensured that she will be out of office after 2015, marking the beginning of the end of a government that she and her late husband, Nestor Kirchner, have led for a decade.

Exit polls suggested the slate led by Massa, the mayor of the wealthy Tigre municipality where many of Argentina's rich and famous live in gated communities, did better in Buenos Aires province than the slate led by Martin Insaurralde, who was hand-picked to represent Fernandez in the lower house.

"Sergio will be the most-voted-for leader in the entire country with this election. This is an overwhelming response by the people to our times," said Dario Giustozzi, who also appeared likely to take a seat in Congress as part of Massi's Renewal Front.

"This is the end of an era, a new space. Now the people have a place where they can be heard," Giustozzi said.

Voting is mandatory in Argentina, and more than 75 percent of the 30 million registered voters cast ballots, Interior Secretary Florencio Randazzo said.